chalcedony

chalcedony (kălsĕdˈənē) [key] [from Chalcedon], form of quartz the crystals of which are so minute that its crystalline structure cannot be seen except with the aid of a microscope. Chalcedony has a waxy luster and is translucent to transparent. The name chalcedony is applied more specifically to white, gray, blue, and brown varieties. Some varieties, differing in color because of the presence of impurities, are agate, bloodstone, carnelian, chrysoprase, jasper, onyx, sard, and sardonyx.